Method of electroplating utilizing zinc dross anodes

ABSTRACT

A method of electroplating steel with a thin layer of zinc utilizing anodes cast from zinc dross taken from the zinc bath of a commercial galvanizing line.

0 United States Patent 11 1 1111 3,891,518 Burwick June 24, 1975 {5 METHOD OF ELECTROPLATING 2.412.543 12/!946 Tanner 204/35 R 2,63l,95l 3/1953 Chester 204/35 R UTILIZING ZINC DROSS ANODES 3.629.078 12/1971 Okada et =11 204/35 R [75] Inventor: Edward R. Burwick, Miamisburg,

Ohio OTHER PUBLICATIONS [73] Assignee: Armco Steel Corporation, Modern Electroplating, Edited by Frederick Lowen- Middletown, Ohio heim 2nd Ed., 1963, pgs. 407-409. [22] Filed: Oct. 16, I974 T f I Primary Examiner- M. Tu arie lo [21] Appl 515353 Attorney, Agent, or Firm-Melville, Strasser, Foster &

Hoffman [52] US. Cl 204/35 R; 204/28; 204/292 [5|] Int. Cl. C23h 5/50; C23b 5/58; BOIk 3/06 [58] Field of Search 204/291, 29 2, 293, 55 R, [57] ABSTRACT 204/55 35 R A method of electroplating steel with a thin layer of zinc utilizing anodes cast from zinc dross taken from References c'led the zinc bath of a commercial galvanizing line. UNITED STATES PATENTS 0 1953.758 4/1934 Hogaboom 204/292 8 Clam, 2 Drawing F'gures 20 2/ 22 PLATING PRE- CHROME; STOCK n- PHOSPHATING RINSE. ACID -I- TANK PHOSPHATI N6 RI M 6E PATENTEI] JUN 24 I975 V Z3; J0 20 2/ 22 u PLATING PR 5- CHROMIC STOCK PI-IOSPHATING R\N5E. ACID TAN K PHOSPHATI N6 R l N SE 1 METHOD OF ELECTROPLATING UTILIZING ZINC DROSS ANODES BACKGROUND OF THE INVENTION This invention has to do with electroplating and has to do more particularly with zinc electroplating of steel strip as an incident of the formation of a zincphosphate coating which provides a surface to which paint will tenaciously adhere. In the formation ofa conventional zincphosphate coating, the strip or sheet to be coated is first conveyed through a plating tank con taining an acid coating bath, usually comprising zinc sulfate. Zinc anodes are suspended in the bath above and below the path of travel of the stock, and coppercovered pinch rolls are positioned at the entry and exit sides of the tank to make electrical contact with the steel strip, which serves as the cathode. Passage of the stock through the tank results in a electrodeposited zinc coating usually having a thickness of about 0.010 to 0.015 oz/ft total for both sides of the stock. Upon passage from the plating tank, the stock is subjected to a heated water rinse which is usually followed by a second rinsing step which includes a nucleating agent to prepare the electroplated surface for the phosphate treatment. The phosphate treatment comprises passing the stock through a treatment tank containing a heated solution of zinc phosphate, whereupon the stock is again rinsed and treated with a final chromic acid rinse.

Heretofore the anodes used in the electroplating operation have been made from essentially pure zinc, and it has been the industry-wide belief that high purity zinc anodes are required for electroplating. Thus, as stated in Principles of Electroplating and Electroforming, by Blum and Hogaboom, published by McGraw-Hill Book Company, Inc., 3rd Edition, 1949, there are three principal grades of zinc that might be used as anodes, known as l) prime Western spelter, with up to 98.5% of zinc; (2) intermediate grade, 99.5 to 99.8 percent of zinc; and (3) high purity, with 99.95 to 99.98 percent of zinc. These grades are also set forth in Metals Handbook, Volume 2, 8th Edition, published by the American Society for Metals, which indicates that best plating results are obtained using the high purity grade, which is stated to have a typical composition of 99.993

ZII'IC.

In contrast to the foregoing, it has now been discovered that zinc electroplating, particularly where used in the formation of a zinc-phosphate coating, can be conducted utilizing anodes having a significant lower zinc content than heretofore was believed possible. More particularly, it has been found that such anodes may be formed from either the top or bottom dross taken from the zinc bath of a commercial galvanizing line.

SUMMARY OF THE INVENTION In accordance with the present invention, the dross, which may be either skimmed from the top of the galvanizing tank or dredged from the bottom, is cast into the desired anode shape without additional treatment. Consequently, zinc electroplating can be conducted utilizing anodes which are composed of material which otherwise would have only scrap value, thereby effecting a very substantial savings in the cost of the anodes. At the same time it has been found that zinc coatings electrodeposited from dross anodes are comparable in quality to coatings formed using the much higher priced essentially pure zinc anodes.

The zinc dross anodes may be of any desired shape, such as bars, plates or balls, depending upon the nature of the electroplating equipment. The equipment itself is of standard character, with the zinc dross anodes substituted for the previously used essentially pure zinc anodes. The plating solution is also conventional, and may comprise a straight sulfate bath or a sulfate bath buffered with minor amounts of other materials, such as sodium chloride and sulfuric acid. It has been found that where the baths include sulfuric acid, some of the zinc is reacted with the sulfuric acid, forming zinc sulfate, and consequently less zinc sulfate is required to maintain the desired solution strength. Typical dross compositions will contain from 88-98 percent zinc and will also include varying amounts of aluminum and iron, usually from about l to 2.5 percent aluminum and from about I to 5 percent iron, together with small quantities of up to 1 percent lead, small quantities of oxygen, usually about 1 percent, and trace amounts of residuals, such as cadmium, tin and antimony. The quality of the electroplated zinc coating does not ap pear to be adversely affected by variations in the composition of the dross within the limits noted, and no appreciable difference has been noted in the quality of the coatings as compared with electrodeposited coatings of substantially pure zinc.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustrating the steps in a conventional zinc-phosphate coating operation.

FIG. 2 is a diagrammatic vertical sectional view of a plating tank equipped with zinc dross anodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. I of the drawings, which illustrates a conventional zinc-phosphate coating line, the stock 10, which will comprise strip or sheets of cold rolled steel, is fed to the plating tank 11 using conventional entry feeding means, such as a sheet conveyor or coil payoff reel.

As seen in FIG. 2, the strip 10 is conveyed through the plating tank 11 by sets of leading pinch rolls 12, I2 and trailing pinch rolls 13, 13, the rolls l2 and I3 being copper-covered and serving to induce current into the strip 10 which becomes the cathode, as indicated by the negative bussbars 14. A series of upper anodes, one of which is indicated at 15, overlie the strip I0 and extend in side-by-side relation between the leading and trailing sets of pinch rolls. Similarly, a series of lower anodes one of which is indicated at I6, underlie the strip 10, the anodes being connected by positive bussbars, indicated at 17, to a generator or other source of direct current. The anodes lie in close proximity to the path of travel of the strip 10 and are placed in such a manner as to allow for uniform zinc plating across the strip. The tank 1 1 contains a plating solution which is basically water with zinc sulfate and sulfuric acid, usually having a Baume of about 25 and pH of about 2. A holding tank 19 is provided to contain the overflow of the plating solution, the overflow being screened and fed back into the plating tank.

Referring again to FIG. 1, following the passage of the stock through the plating tank, the stock is subjected to a pre-phosphating treatment 20 during which it is subjected to one or more rinsing operations. In a typical arrangement, the stock is first subjected to a water rinse at about l40F. in a water rinse tank in which the stock passes between top and bottom sprays in the tank, whereupon it is passed through a second rinse tank containing water and a nucleating agent to prepare the plated surfaces for accepting the phosphating treatment. The second water rinse tank operates at a temperature of from 120F. to 145F. depending upon the material being coated.

Following the pre-phosphating operation, the stock enters the phosphating unit 21 wherein it passes through a solution of zinc phosphate which is heated by steam or other suitable means to maintain a bath temperature at about 140F. Where the stock is to be surfaced treated only, it is carried across the top of the phosphating tank by rubber rolls. As the strip exits the phosphating tank, it passes through a rinse tank 22 wherein it is subjected to a heated water rinse to remove the excess phosphate solution, whereupon the stock passes through a covered chromic acid tank 23 wherein it is subjected to a final rinse at about l70F. in a solution of chromic acid of about 2 pH. Upon leaving the chromic acid tank 23, the stock will be collected by a suitable coiling mandrel or sheet catcher, and if desired, it may be subjected to additional operations, such as roller leveling or oiling.

In accordance with the present invention, the plating operation conducted in the plating tank 11 utilizes anodes l and 16 formed from zinc dross removed from the galvanizing tank of a conventional galvanizing line, whether it be a hot-dip line, a continuous line, or a wire coating operation. Either top or bottom dross may be used, with one exception, which is top dross from a flux operation wherein a relatively thick layer of flux is floated on the zinc bath. In a typical galvanizing operation the top dross forms at a rate of about four pounds per ton of sheet processed and is removed about every four hours. Bottom dross, on the other hand, accumulates at a much slower rate and is usually removed at relatively infrequent intervals, usually in a dredge manner during shutdown.

Upon removal, the dross is poured directly into open molds of the desired size and configuration to form the anodes. For example, the top dross may be skimmed from the top of the zinc bath using a six inch diameter spoon having holes to permit the zinc to flow back into the bath. The removed dross is spooned into the molds and compacted sufficiently to form a solid mass, although it may be noted that due to the physical nature of the dross, the anodes exhibit visible porosity. However, this characteristic does not appear to have any bearing on the performance of the anodes since it has been found that the same size anodes of pure spelter and dross performed essentially the same.

With the exception of top dross from a flux coating bath, the composition of the dross from which the anodes are cast does not appear to have any material effeet on their performance. While top dross tends to be rich in aluminum, whereas bottom dross tends to be rich in iron, both perform satisfactorily and without appreciable difference as compared with essentially pure zinc anodes. This is believed due to the fact that the elements other than zinc are not required for the plating operation. Accordingly, as used herein and in the claims which follow, the term zinc dross" contemplates either top or bottom dross containing up to 2.5 percent aluminum, 5 percent iron, 1 percent lead, and about 1 percent oxygen, together with trace amounts of residuals, such as cadmium, tin and antimony, the balance being zinc. This chemistry in itself rules out top dross from a flux coating bath since it will contain substantial quantities of flux and materially less zinc.

The plating solution 18 employed in the plating tank 11 is of conventional character and normally will comprise an acid bath containing zinc sulfate, sodium chloride and sulfuric acid. Where such a bath is employed using zinc dross anodes, it has been observed that more zinc is reacted with the sulfuric acid, forming zinc sulfate, and consequently less zinc sulfate is required to maintain the bath strength. In practicing the invention, conventional current densities will be utilized for the plating operation, the rate of deposition being a function of current density. The bath temperature will also be conventional, usually about F. for acid baths, although temperatures as high as F. may be employed. No significant bath contamination problems have been encountered utilizing the zinc dross anodes.

Zinc dross anodes may be utilized to electroplate up to about one ounce per square foot of zinc, which amount of zinc would be suitable for substantially any electroplating requirement. However, electroplating is not normally economically feasible for heavier thicknesses, and for the most part, particularly in the formation of zinc-phosphate coatings, the zinc thickness will be from about 0.0l0 to 0.015 ounces per square foot total weight for both sides of the sheet or strip stock.

Zinc-phosphate stock coated using zinc dross anodes has been found comparable in quality to stock coated using essentially pure zinc anodes when painted either with indoor or outdoor paint under standard ASTM testing procedures. For example, when subjected to standard salt spray and water soak tests, no appreciable differences were noted in creepage, blistering or adhesion. Similarly, the plating solutions were tested to determine if any solution contamination problems existed using zinc dross anodes, and none was found to exist.

As should now be apparent, the instant invention provides a method of electroplating utilizing zinc dross anodes as a replacement for the substantially pure zinc anodes heretofore thought to be mandatory for electroplating zinc. The zinc dross anodes have been found to be effective as substantially pure zinc anodes for electroplating purposes, and at the same time the anode cost is materially reduced in that the cost of dross anodes is approximately one-third the cost of prime Western spelter, which is the least expensive of the grades conventionally used for anodes.

Modifications may be made in the invention without departing from its spirit and purpose; and various modifications have been set forth and others will occur to the worker in the art upon reading this specification. The base material being treated is normally cold rolled steel, which may be of commercial quality, drawing quality or special killed drawn quality, or it may comprise stainless steel if it is desired to treat it for paintability.

The embodiments of the invention in which an exlcusive property or privilege is claimed are defined as follows:

1. In a method of electrodepositing zinc on steel wherein the steel stock is passed through a plating tank between zinc anodes submerged in an aqueous plating solution, with the stock acting as a circuit forming cathode, and an electrical charge is imposed through the anodes to effect plating, the improvement which comprises the step of plating the stock with zinc utilizing anodes consisting of zinc dross.

2. The method claimed in claim 1 wherein the zinc dross anodes comprise zinc dross skimmed from the top of a zinc galvanizing tank which is free from a surface layer of flux and poured directly into anodeforming molds.

3. The method claimed in claim 1 wherein the zinc dross anodes comprising dross taken from the bottom of a zinc galvanizing tank and poured directly into anodeforming molds.

4. The method claimed in claim 1 including the step of utilizing a water solution of zinc sulfate containing sodium chloride and sulfuric acid as the aqueous plating solution.

5. In a method of forming a zinc-phosphate coating on steel stock wherein the stock is first passed through a plating tank between zinc anodes submerged in an aqueous plating solution, with the stock acting as a circuit forming cathode, and an electrical charge is imposed through the anodes to effect plating of a zinc coating on the stock, the zinc plated stock then being rinsed and passed through an aqueous solution of zinc phosphate, followed by rinsing and treatment with chromic acid to form a zinc-phosphate coating thereon, the improvement which comprises the step of plating the stock with zinc utilizing anodes consisting of zinc dross.

6. The method claimed in claim 5 wherein the zinc dross anodes comprise zinc dross skimmed from the top of zinc galvanizing tank which is free from a surface layer of flux and poured directly into anode-forming molds.

7. The method claimed in claim 5 wherein the zinc dross anodes comprise zinc dross taken from the bottom of a zinc galvanizing tank and poured directly into anodeforming molds.

8. The method claimed in claim 5 including the step of zinc plating the stock in a water solution of zinc sulfate containing sodium chloride and sulfuric acid. 

1. IN A METHOD OF ELECTRODEPOSITING ZINC ON STEEL WHEREIN THE STEEL STOCK IS PASSED THROUGH A PLATING TANK BETWEEN ZINC ANODES STBMERGED IN AN AQUEOUS PLATING SOLUTION, WITH THE STOCK ACTING AS A CIRCUIT FORMING CATHODE, AND AN ELECTRICAL CHARGE IS IMPOSED THROUGH THE ANODES TO EFFECT PLATING, THE IMPROVEMENT WHICH COMPRISES THE STEP OF PLATING THE STOCK WITH ZINC UTILIZING ANODES CONSISTING OF ZINC DROSS.
 2. The method claimed in claim 1 wherein the zinc dross anodes comprise zinc dross skimmed from the top of a zinc galvanizing tank which is free from a surface layer of flux and poured directly into anode-forming molds.
 3. The method claimed in claim 1 wherein the zinc dross anodes comprising dross taken from the bottom of a zinc galvanizing tank and poured directly into anodeforming molds.
 4. The method claimed in claim 1 including the step of utilizing a water solution of zinc sulfate containing sodium chloride and sulfuric acid as the aqueous plating solution.
 5. In a method of forming a zinc-phosphate coating on steel stock wherein the stock is first passed through a plating tank between zinc anodes submerged in an aqueous plating solution, with the stock acting as a circuit forming cathode, and an electrical charge is imposed through the anodes to effect plating of a zinc coating on the stock, the zinc plated stock then being rinsed and passed through an aqueous solution of zinc phosphate, followEd by rinsing and treatment with chromic acid to form a zinc-phosphate coating thereon, the improvement which comprises the step of plating the stock with zinc utilizing anodes consisting of zinc dross.
 6. The method claimed in claim 5 wherein the zinc dross anodes comprise zinc dross skimmed from the top of zinc galvanizing tank which is free from a surface layer of flux and poured directly into anode-forming molds.
 7. The method claimed in claim 5 wherein the zinc dross anodes comprise zinc dross taken from the bottom of a zinc galvanizing tank and poured directly into anodeforming molds.
 8. The method claimed in claim 5 including the step of zinc plating the stock in a water solution of zinc sulfate containing sodium chloride and sulfuric acid. 